The overall goal of this research proposal is to functionally characterize MEF2B, a transcription factor that has been recently identified as a master regulator of the germinal center (GC) reaction and whose genomic locus is targeted by activating mutations in Diffuse Large B Cell Lymphoma (DLBCL) and Follicular Lymphoma (FL), the two most common forms of human lymphoma. The MEF2B gene encodes a transcriptional activator and is found mutated in ~11% of diffuse large B-cell lymphomas (DLBCL) and ~12% of follicular lymphomas. We have shown that MEF2B directly activates the transcription of the proto-oncogene BCL6 in normal germinal center (GC) B cells and is required for DLBCL proliferation. MEF2B mutations enhance MEF2B transcriptional activity either by disrupting its interaction with the co-repressor CABIN1, or by rendering it insensitive to the inhibitory effects f PKA-mediated phosphorylation and sumoylation. Consequently, BCL6 transcriptional activity is deregulated in DLBCL harboring MEF2B mutations. Based on these initial results, this research proposal aims at investigating the role of MEF2B as modulator of GC development and lymphomagenesis. In particular, the following specific aims will be pursued: 1) determine the role of MEF2B in normal GC development by i) analyzing the phenotype of mice carrying conditional, GC-specific inactivation of MEF2B, ii) identifying the MEF2B target genes by ChIP-seq, and iii) the proteins that regulate MEF2B activity in GC B cells by mass spectrometry analysis; 2) functionally characterize the MEF2B mutations identified in DLBCL and FL, including both the N-terminus missense mutants that affect transcriptional co-repressor binding and the C- terminus truncating mutations that affect its negative regulation by phosphorylation and/or sumoylation; 3) determine the consequences of MEF2B mutations in vivo by analyzing transgenic mice with conditional activation of mutant MEF2B alleles in GC B cells, alone or in combination with other mutations co-existing in DLBCL and FL.